Izvestiya, Physics of the Solid Earth最新文献

Abstract—The paper analyzes the results of shallow geophysical studies and seismicity of the Selenga Delta and adjacent areas of the South Baikal Basin, where the destructive Tsagan earthquake of 1862 with M7.5 occurred. This study area is characterized by the formation of epicentral zones, which overlap each other in a en echelon–like manner, as well as the seismic “quiescence” of the eastern segment of the Delta Fault, to which the dislocations of the Tsagan earthquake are confined. The distribution of seismicity and similarity of coseismic effects during the Tsagan and Middle Baikal (1959, M6.8) earthquakes, allow us to suggest that the 1862 shock was caused by displacement along a fault within the water area, either along the lateral Coastal Fault or along the intradepression Middle Baikal Fault, the length of which allowed an earthquake with M > 7 to occur. The activation of one of these faults in 1862 led to subsidence of the northeastern block of the Delta Trough with the opening of faults along its perimeter, including along the Delta Fault. The consequence of activation of the medium after strong events and clustering of strong shocks in time is a large number of weak shocks, reflected by the increased value of the slope of the recurrence graph (γ = –0.54 ± 0.01). According to geophysical data, the range of vertical movements during the last seismotectonic activation cycle reached 35 m.

Abstract—When studying the distribution of earthquake epicenters in the Northern Tien Shan region, a tendency was discovered for weak seismic events to cluster in relatively small areas. Understanding the conditions under which earthquakes occur and how they are grouped requires analysis of various geophysical features in conjunction with seismic event data. Using the Kohonen neural network, which allows for multidimensional classification of data, clusters were identified based on different sets of characteristics of the geophysical medium directly in the space of geographic coordinates. A comparison was made between cluster maps obtained using one or several geophysical parameters simultaneously and data on seismic events in the region under study. This integrated analysis revealed that there is a spatial correlation between clusters formed by a combination of parameters such as specific electrical resistance (electrical resistivity), temperature, vertical gradient of the ratio of compressional and shear seismic wave velocities, and the distribution of seismic events in the area considered.

Abstract—Discrete mathematical analysis (DMA) is a method of analyzing discrete data that actively uses fuzzy mathematics (FM) and fuzzy logic (FL), allowing it to take into account the opinion and experience of a researcher in its work to a much greater extent than classical methods. The latest results of DMA on the study of discrete functions using FL, clustering in complex multidimensional arrays, as well as a constructive description of the diversity of solutions of systems of linear algebraic equations (SLAE) allow us to propose a new version of geophysical integration, in which FM and FL, and therefore the researcher, play a major role. The work presents the concept of geophysical integration, which is based on the theoretical apparatus of DMA, FM, and FL and practical, geophysical results of the application of DMA as evidential examples.

Abstract—We propose a new approach to constructing a two-dimensional (2D) temperature model of the Earth’s interior based on solving the steady-state heat conduction equation with varying coefficients. To this end, using seismic velocity data measured on the sublatitudinal profile in the Northern Tien Shan, we built the lithotype model which was then employed to estimate the 2D distribution of thermal conductivity of rocks across the section with the allowance for its dependence on background temperature.

Coseismic displacements and deformations of the Earth’s crust from the world’s strongest earthquakes spread to significant distances from their epicenters. Assessment of the impact of earthquakes on the characteristics of movements and deformations of the crust in remote areas is of great scientific and practical interest. The article analyzes the impact of the earthquake in Kahramanmaraş, Turkey (February 6, 2023) on movements and deformations of the crust of the Caucasus region based on time series of continuous GNSS observations. The velocities of movements in the region before and after the Kahramanmaraş earthquake were determined. The results show the appreciable (and statistically significant) impact of earthquakes on horizontal and vertical crustal movements in remote areas. The clockwise reversal of the horizontal motion velocity vectors, decrease in horizontal velocities, and, predominantly, an increase of vertical motions were confidently recorded. The general direction of horizontal motions of GNSS stations in the Caucasus agrees with the postseismic velocity vector of ANTP station, closest to the Kahramanmaraş epicentral zone, agrees with the displacement trend of the Arabian Plate. The general trends of combined of pre- and postseismic motions show the effect of mountain structures of the Lesser Caucasus counteracting the northeastern pressure of the wedge of the Arabian Plate. The results provide important information to explain the mechanisms of regional actuotectonics and the role of strong seismic events in the modern crustal movement regime of the Caucasus.

Abstract—Paleointensity of the geomagnetic field is determined from the natural remanent magnetization of Holocene basalts from the Red Sea rift zone by the Thellier method in the Coe modification with heating in air and argon. It is shown that when basalts are heated in air, titanomagnetite, the carrier of remanent magnetization, undergoes mineralogical changes at temperatures 50–100°C below the maximum blocking temperature, which impedes paleointensity determination. It is established that the Thellier–Coe procedure conducted in argon atmosphere avoids oxidation of titanomagnetite and thus significantly improves the quality of paleointensity determination. When heating is conducted in air, only 50–60% of the natural remanent magnetization can be used to calculate paleointensity, while when in argon, the percentage reaches up to 87% and the quality factor is 1.5–3 times higher. The paleointensity value estimated from heating in argon is 10–20% lower than that estimated from heating in air. The paleointensity of 62 ± 1 µT determined from the Red Sea basalts is about 1.7 times as high as the current field in the region according to the IGRF13 model. The high paleointensity value allows us to attribute the period of formation of the studied basalts to the time of the Levantine Iron Age Anomaly 1.5–0.5 ka BC.

Abstract—The problem of modeling a nonstationary electric field in a spatially inhomogeneous medium with three closely located conductive ore objects is considered for an installation in which the generator loop covers the entire area with the ore objects and measurements are carried out inside it. Based on numerical modeling data, their influence on the emf measured using the transient processes method was studied. Analysis of the received signals showed that the significant influence of conductive objects is local in nature with time and manifests itself in the zone of projection of objects onto the Earth’s exposed surface. The magnitude of the emf anomaly and its duration correlates with the number of conducting objects.

Abstract—By the mid-2000s, a large body of magnetostratigraphic evidence had been accumulated, suggesting the existence of a geomagnetic superchron of reversed polarity in the Early to Middle Ordovician (the Moyero Reversed Polarity Superchron). The subsequent studies generally supported this hypothesis. However, there were also isolated reports that could either indicate the presence of a fine structure in the superchron, i.e., the existence of several short intervals of normal polarity within the superchron, or significantly limit its duration. Meanwhile, study of geomagnetic superchrons, further substantiation of the very existence of the Ordovician superchron and establishing its time limits is crucial for the development of the physical theory of geomagnetism, geological correlation, etc. Therefore, any information related to this issue should be carefully considered and verified. This paper presents the results of the detailed magnetostratigraphic study of some intervals from several Ordovician Ladoga sections. These intervals are coeval to those from the Ordovician of the northwestern Russia, Baltic region, and Scandinavia whose age corresponds to the Moyero reversed polarity superchron but which were previously reported to contain separate levels of normal polarity. Combined with the recent data on the Mishina Gora section of the Pskov region, these results provide yet another significant argument supporting the existence of the Moyero Ordovician geomagnetic superchron and its lack of fine structure.

Abstract—The paper evaluates the influence of secondary magnetization on the determination of characteristic component and paleointensity in basalt rocks whose magnetic remanence is carried by titanomagnetite with varying degrees of oxidation. To this end, laboratory experiments on magnetization acquisition were performed on basalt samples from the submerged Reykjanes Ridge, North Atlantic. The “primary” total thermoremanent magnetization TRM was imparted to rock samples by their exposure at temperature of 600°C followed by cooling in a magnetic field of 50 µT. The “secondary” chemical remanent magnetization CRM, perpendicular to TRM, was overprinted by subsequent exposure of samples at 350°C for 200 h in the same field. Directly at a temperature of 350°C, temporal monitoring of the changes in the TRM and CRM components was carried out. The samples with laboratory-induced total remanent magnetizations were subjected to the Thellier-type paleointensity experiments, and the results were used to construct the Arai–Nagata and Zijderveld diagrams. It is shown that CRM is formed on the new ferrimagnetic phases that resulted from the oxidation of an unstable fraction of titanomagnetite, and the CRM intensity critically depends on the degree of oxidation of the initial material. All the constructed diagrams have linear segments with different slopes, which are identified in the low-temperature (LT) range from 20–350 to 450°C, medium-temperature (MT) range from 450–475 to 500–530°C, and high-temperature (HT) range from 500–530 to 560–600°C. It is shown that the presence of a secondary component leads to errors in both paleodirection and paleointensity determinations from the primary component, and the errors are greater the greater the secondary CRM relative to the primary TRM. A new criterion of reliability of paleomagnetic data is proposed. This criterion applies to both paleointensity and paleodirection and states that the primary component should be several times as large as the secondary component.

Abstract—The influence of the magnitude of a laboratory magnetic field in the Thellier–Coe method on the results of determining the archaeointensity of the magnetic field was studied. The experiments were carried out on the same samples of artificial Neolithic ceramics and medieval bricks from Veliko Tarnovo city in laboratory magnetic fields of 20, 35, 50, 75, and 100 μT. The results show that for the studied samples, the magnitude of magnetic field induction determined in the Thellier–Coe experiments (archaeointensity) depends on the magnitude of the laboratory magnetic field induction in the Thellier–Coe method (which increases approximately according to a quasi-logarithmic law). It is suggested that the reason for the obtained dependence is the nonlinear dependence of the thermoremanent magnetization on the magnitude of the magnetic field (in the range of 20–100 μT) in which it was created.